A gas turbine engine contains a compressor in fluid communication with a combustion system, where the combustion system often contains a plurality of combustors arranged about the engine. The compressor raises the pressure of the air passing through each stage of the compressor and directs it to the combustors where fuel is injected and mixed with the compressed air. The fuel and air mixture ignites and combusts creating a flow of hot gases that are then directed into the turbine. The hot gases drive the turbine, which in turn drives the compressor, and for electrical generation purposes, also drives a generator.
Most combustion systems utilize a plurality of fuel injectors for staging, emissions purposes, and flame stability. A common practice to ensure flame stability is to employ a pilot fuel nozzle whose main purpose is to supply a rich fuel stream to the combustion zone. An example of a pilot fuel nozzle of the prior art is shown in FIG. 1. A combustor 10 contains a liner 11 having a combustion zone 12, and is enclosed by an end cover 13, where end cover 13 includes a pilot fuel nozzle 14 and a plurality of main fuel nozzles 15 arranged in a radial array about pilot fuel nozzle 14. Pilot fuel nozzle 14 is generally tubular in shape having a first outer diameter 16 and a first inner diameter 17 throughout, thereby forming a wall thickness 17A. Fixed to pilot fuel nozzle 14 is a nozzle tip 18 having a plurality of fuel injection holes (not shown) for injecting a fuel from pilot fuel nozzle 14 into combustion zone 12.
Pilot fuel nozzle 14 is fixed to end cover 13, cantilevered from flange 19. Field experience with a pilot fuel nozzle of the configuration shown in FIG. 1 includes severe fractures that have liberated portions of nozzle 14 causing significant damage to the  combustor and turbine sections of the engine, forcing engine shutdown and extensive repairs.
Prior art fuel nozzles have incorporated devices such, as the one shown in U.S. Pat. No. 6,038,862, to address the issue of high cycle fatigue. Unfortunately, this device cannot be readily applied to a fuel nozzle comprised of a single tube cantilevered in a combustor since the fuel nozzle structure is not configured to utilize a damper. A cost effective alternate means to address this problem is necessary.
The present invention seeks to overcome the shortfalls of the prior art pilot fuel nozzle configuration by providing a pilot fuel nozzle structure that has a natural frequency well outside of a known operating range of the gas turbine engine such that cracks and fractures due to resonance are avoided and potential damage to the combustor and turbine sections are minimized. A cost effective method for changing the natural frequency of existing pilot fuel nozzles is also disclosed such that existing pilot fuel nozzles can be reconfigured, thereby avoiding scrap hardware.